Reference may be made to the following U.S. Pat. Nos. of interest: 3,047,808; 3,381,230; 3,475,689; 3,529,245; 3,891,840; 3,119,984; 3,476,924; 3,512,140; 3,541,320; 3,541,446; 3,566,265; 2,667,055; 3,714,591; 3,778,725; 3,784,919; and 3,942,172.
Electronic integrating circuits commonly employ an amplifier with an integrating capacitor connected in parallel between the amplifier output and input terminals. Due to a phenomena known as "dielectric absorption" occurring in the integrating capacitor, the integration results are subject in a small amount of error. In some circumstances the integration error is tolerable. However in systems requiring integration precision, it is not only desirable but necessary to remove or substantially eliminate the error due to dielectric absorption.
Dielectric absorption is a fundamental property of all capacitors in which a small percentage of the energy stored during integration is not immediately recoverable upon discharge of the capacitor, but rather is released at a rate governed by the physical characteristics of the dielectric material forming the capacitor. This phenomena is believed to occur as a result of space charge polarization within the capacitor's dielectric material leading to an apparent increase in capacitance. The macroscopic effects of dielectric absorption are most commonly observed as a residual error voltage build-up across the previously discharged integrating capacitor. The error voltage increases to a maximum value, linearly related to the voltage across the integrating capacitor prior to discharge. Upon reaching this maximum value the error voltage then slowly declines at a rate governed by the internal and external shunting impedances. The occurrence of such a residual error voltage build-up in a desired precision integrating circuit can seriously limit the integration resolution and accuracy.
One attempt to lessen the effect of dielectric absorption is to utilize high quality capacitors for the integrating capacitor, however such desirable capacitors are more expensive and occupy a volume an order of magnitude greater than a comparable capacitor of lesser quality. An alternative solution is to allow sufficient time between integrating periods to insure that the dielectric absorption voltage has declined to zero. In many cases this is not a viable solution, particularly where the integration circuit is to first integrate a large amplitude signal prior to reset and subsequently integrate a relatively small amplitude signal.